1. Field of the Invention
The invention relates to a system of mirrors for improving the quality of an azimuthally polarized beam. More specifically, it relates to mirrors that transform an azimuthally polarized cylindrical beam into sections that are substantially equal in phase and polarization.
2. Description of the Prior Art
Certain modern laser concepts have a gain media which fills a cylindrical annulus. In order to produce an annular-shaped electromagnetic mode, resonators have been devised which use a pair of conical mirrors called an axicon. The axicon, which converts a cylindrical beam to an annular beam and vice versa will, in general, affect the polarization of the beam and resonator mode. In many caes, an electromagnetic mode with azimuthal polarization results. In other words, the electric vector is everywhere oriented tangential to a circle centered on the axicon axis.
This azimuthally polarized beam, by virtue of the symmetry of the electric vectors, will have zero intensity on axis regardless of how far it propagates. For high energy lasers, this zero intensity on axis is desirable inside the resonator because it protects the axicon tip from an otherwise high intensity. However, once the beam exits the laser it continues, by symmetry, to have a dark spot of zero intensity at its center so as to reduce the intensity on any work piece or target onto which the beam is focused or otherwise directed.
It has been suggested ("Polarization Effects of Axicons", Applied Optics, Vol. 18, No.5, Mar. 1, 1979, by David Fink) that axicons might be designed to alter the mode structure inside the resonator. Methods of coating axicons inside resonators have been developed that avoid the azimuthally polarized mode. These changes in the polarization of the resonator mode improve the quality and behavior of the output beam. However, these changes in polarization are accompanied by a very large increase in intensity on the intracavity axicon tip, threatening its destruction in a high energy device.
It would be desirable to be able to retain the azimuthal polarization inside the resonator (with its low axicon tip intensity) but convert the polarization of the output beam to linearly or circularly polarized light. Heretofore, no technology to achieve this was known.